Noninvasive method and system for measuring pulmonary ventilation

ABSTRACT

A pulmonary ventilation system comprising means for storing an empirical relationship that is designed and adapted to determine at least one pulmonary ventilation parameter as a function of a plurality of measured anatomical distances and volume-motion coefficients, means for acquiring the anatomical distances, means for determining the plurality of motion coefficients, and processing means for determining the ventilation parameter based on the acquired anatomical distances and determined plurality of volume-motion coefficients. In one embodiment, the system further includes means for acquiring base-line ventilation characteristics and means for correlating the base-line ventilation characteristics to the ventilation parameter determined with the empirical relationship.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/231,692, filed Sep. 5, 2008, now U.S. Pat. No. 8,790,273, which isincorporated herein by reference in its entirety.

FIELD OF THE PRESENT INVENTION

The present invention relates generally to measuring respiratory airvolume in humans. More particularly, the invention relates to anoninvasive method for quantitatively measuring pulmonary ventilationand a ventilation system employing same.

BACKGROUND OF THE INVENTION

As is well known in the art, in medical diagnosis and treatment, it isoften desirable to quantitatively measure over time the respiratory airvolume or pulmonary ventilation. This has conventionally been done byhaving the patient or subject breathe into a mouthpiece connected to aflow rate measuring device. Flow rate is then integrated to provide airvolume change.

There are, however, several drawbacks and disadvantages associated withemploying a mouthpiece. A mouthpiece is difficult to use for long termsubject monitoring, especially for ill, sleeping or anesthetizedsubjects. Further, it is uncomfortable for the subject, tends torestrict breathing, and is generally inconvenient for the physician ortechnician to use.

As is also well known in the art, there are qualitative respirationmonitors available that do not require a mouthpiece. Illustrative arethe systems disclosed in U.S. Pat. Nos. 3,831,586 and 4,033,332.Although the noted systems eliminate most of the disadvantagesassociated with a mouthpiece, the systems do not, in general, provide anaccurate measurement of air volume. Further, the systems are typicallyonly used to signal an attendant when a subject's breathing activitychanges sharply or stops.

Another means for quantitatively measuring respiratory or lung volume isto measure the change in size of the rib cage and abdomen, as it is wellknown that lung volume is a function of these two parameters. A numberof systems and devices have been employed to measure the change in size(i.e. Δ circumference) of the rib cage and abdomen, including mercury inrubber strain gauges, pneumobelts, magnetometers, and respiratoryinductive plethysmograph (RIP) belts, see, e.g., D. L. Wade, “Movementsof the Thoracic Cage and Diaphragm in Respiration”, J. Physiol., pp.124-193 (1954), Mead, et al, “Pulmonary Ventilation Measured from BodySurface Movements”, Science, pp. 196, 1383-1384 (1967).

In practice, respiratory magnetometers and RIP belts are primarily usedto measure the change in size of the rib cage and abdomen. As is wellknown in the art, respiratory magnetometers consist of tuned pairs ofelectromagnetic coils or magnetometers; one coil being adapted totransmit a specific high frequency AC electromagnetic field (i.e.transducer) and the other coil (i.e. receiver) being adapted to receivethe field. To measure the anteroposterior diameter of the rib cage, afirst coil, e.g., transducer, is typically placed over the sternum atthe level of the 4th intercostal space and the second coil (of the pair)is placed over the spine at the same level. To measure theanteroposterior diameter of the abdomen, a third coil is typicallyplaced on the abdomen at the level of the umbilicus and a fourth coil(of the pair) is placed over the spine at the same level.

Over the operational range of distances, the output voltage is linearlyrelated to the distance between a pair of coils; provided, the axes ofthe coils or magnetometers remain parallel to each other. As rotation ofthe axes can change the voltage, the transducer and receiver coils mustbe secured to the skin in a parallel fashion and rotation due to themotion of underlying soft tissue that must be minimized.

A potential limitation of the use of such coils or magnetometers ispresented in environments that contain large metal structures orelectric motors. Such devices produce extraneous electromagnetic fieldsand consequently affect the magnetometer voltage output.

RIP belts consist of two loops of wire that are coiled and sewed into anelastic belt. To measure changes in cross-sectional areas of the ribcage and abdomen, one belt is secured around the mid-thorax and a secondbelt is placed around the mid-abdomen.

The voltage change from the belts is generally linearly related tochanges in the enclosed cross-sectional area. When the RIP belts areoperated in the DC-coupled mode, they can detect shifts in chest walldimensions, e.g. a change of FRC. However, the AC-coupled mode istypically preferred for tidal volume measurements.

For quantitative measurements, RIP uses a “two-degrees-of-freedom” modelto assess changes in perimeters (i.e. cross-sectional area) of the ribcage and abdomen. Since the first rib and adjacent structures of theneck are relatively immobile, the moveable components of the thoraciccavity are taken to be the anterior and lateral walls of the rib cageand the abdomen. Changes in volume of the thoracic cavity will then bereflected by displacements of the rib cage and abdomen.

Displacement (i.e. motion) of the rib cage can be directly assessed.Diaphragm displacement cannot be measured directly, but since theabdominal contents are essentially incompressible, caudal motion of thediaphragm relative to the pelvis and the volume it displaces isreflected by outward movement of the anterolateral abdominal wall.

The “two-degrees-of-freedom” model embraced by most in the field holdsthat the volume displacement of the respiratory system, i.e. tidalvolume (V_(T)), is equal to the sum of the volume displacements of therib cage and abdomen, i.e.

V _(T) =αRC+βAb  Eq. 1

where:RC and Ab represent linear displacements of the rib cage and abdomen,respectively; and α and β represent volume-motion coefficients.

As is well known in the art, RC and Ab linear displacements areconverted to RC and Ab volume displacements when multiplied by the α andβ volume-motion coefficients.

It is well established that the use of the noted“two-degrees-of-freedom” model can provide an estimate of V_(T) that iswithin 10% accuracy of ventilation measured at the mouth; provided, thesubject is confined to one body position.

Two different approaches primarily used for determining the necessaryvolume-motion coefficients of the rib cage and abdomen are the isovolumetechnique and the multiple linear regression technique. In the isovolumetechnique, the subject first performs an isovolume maneuver, shiftingvolume back and forth between the rib cage and abdominal compartmentswhile holding the glottis closed, whereby there is no net volume changeof the system. Since V_(T) equals zero, Equation 1 can be modified asfollows:

RC=(−β/α)Ab  Eq. 2

On a graph of rib cage and abdomen signals, the slope of the isovolumeline is equal to the ratio −β/α.

In practice, the gains of the rib cage and abdomen signals are oftenadjusted, whereby the slope of the isovolume line equals one. The ribcage and abdomen displacements are thus equal for any volume change. Thetwo signals can then be directly summed to provide volume.

The isovolume method is based on the assumptions that displacements ofthe surfaces of the rib cage and abdomen are representatively sampled atthe measured location, and are similar during isovolume efforts andspontaneous breathing. Since volume-motion coefficients change withposture, the isovolume calibration must be repeated in each bodyposition.

Computer-assisted regression techniques, such as multiple linearregression, are used to determine volume-motion coefficients by solvinga matrix of multiple simultaneous equations of changes in chest walldimensions and lung volume. An advantage of these techniques is that nospecial calibration maneuver is required to generate volume-motioncoefficients.

A limitation of any approach that uses chest wall motion to assessventilation is, however, that the overall volume change of the chestwall being measured includes not only changes in lung volume, but alsoblood volume shifts into and out of the thoracoabdominal cavity. Thiscan occur when the respiratory system is subjected to large pressurechanges, or with changes in posture (e.g. between supine and uprightposition).

Another limitation is related to distortion that can occur within therib cage or abdomen (e.g. between the upper and lower rib cage orbetween the lower transverse and AP rib cage).

As is well known in the art, the accuracy of “two-degrees-of-freedom”model and, hence, methods employing same to determine volume-motioncoefficients of the rib cage and abdomen, is further limited by virtueof changes in spinal flexion that can accompany changes in posture.Indeed, it has been found that V_(T) can be over or under-estimated byas much as 50% of the vital capacity with spinal flexion and extension,see McCool, et al., “Estimates of Ventilation From Body SurfaceMeasurements in Unrestrained Subjects”, J. Appl. Physiol., vol. 61, pp.1114-1119 (1986); and Paek, et al., “Postural Effects on Measurements ofTidal Volume From Body Surface Displacements”, J. Appl. Physiol., vol.68, pp. 2482-2487 (1990).

There are two major causes that contribute to the noted“two-degrees-of-freedom” model error(s) and, hence, limitation. A firstcontributing cause of the error is due to the substantial displacementof the summed rib cage and abdomen signals that occurs with isovolumespinal flexion and extension or pelvic rotation, which is illustrated inFIG. 1.

These shifts are a consequence of conservation of volume. As one of thethoracoabdominal boundaries is pushed in, another must be pushed out.

The second contributing cause of the error is due to posturally-inducedchanges in volume-motion coefficients. With isovolume spinal flexion,the rib cage comes down with respect to the pelvis and the axialdimension of the anterior abdominal wall becomes smaller. Therefore,less abdominal cavity is bordered by the anterior abdominal wall.

With a smaller anterior abdominal wall surface to displace, a givenvolume displacement of the abdominal compartment would be accompanied bya greater outward displacement of the anterior abdominal wall. Theabdominal volume-motion coefficient would accordingly be reduced.

It has, however, been found that the addition of a measure of the axialmotion of the chest wall, i.e. changes in the distance between thexiphoid and the pubic symphysis (Xi), provides a third degree offreedom, which, when employed to determine V_(T) can reduce the posturerelated error associated with the “two-degrees-of-freedom” model towithin 15% of that measured by spirometry, see Paek, et al., “PosturalEffects on Measurements of Tidal Volume From Body SurfaceDisplacements”, J. Appl. Physiol., vol. 68, pp. 2482-2487 (1990); andSmith, et al. “Three Degree of Freedom Description of Movement of theHuman Chest Wall”, J. Appl. Physiol., Vol. 60, pp. 928-934 (1986).

Smith, et al. proposed the following “three-degrees-of-freedom” model todetermine tidal volume, i.e.

V _(T) =αRC+βAb+γXi  Eq. 3

where:RC and Ab represent linear displacements of the rib cage and abdomen,respectively;Xi represents the Δ distance between the xiphoid and the pubicsymphysis; and α, β and γ represent volume-motion coefficients for RC,Ab and Xi.

Referring to FIG. 2, there are shown graphical illustrations of % errorof estimated V_(T) determined with one, two and three degrees of freedommodels (i.e. x-axis) during various postural movements or maneuvers. Itcan be seen that the use of a “three-degrees-of-freedom” modelincorporating the third independent variable, i.e. the Δ distancebetween the xiphoid and the pubic symphysis (“Xi”), enhances theaccuracy with which volume is estimated from body surface motion inthose maneuvers that incorporate changes in spinal attitude.

There are, however, similarly several drawbacks and disadvantagesassociated with the “three-degrees-of-freedom” model. A major drawbackis that the “three-degrees-of-freedom” model reflected in Eq. 3 above isstill limited in accuracy to about 15% of actual ventilation inindividuals who are doing freely moving postural tasks, such as bending,sitting or standing, due to spinal flexion.

It would thus be desirable to provide an improved method and associatedsystem for determining tidal volume (or pulmonary ventilation) thatsubstantially reduces or eliminates the drawbacks and disadvantagesassociated with conventional methods and systems that are employed todetermine pulmonary ventilation.

It is therefore an object of the present invention to providenoninvasive methods and associated systems for determining pulmonaryventilation that substantially reduce or eliminate the drawbacks anddisadvantages associated with conventional methods and systems fordetermining pulmonary ventilation.

It is another object of the invention to provide noninvasive methods andassociated systems for determining pulmonary ventilation thatsubstantially reduce the accuracy errors associated with conventionaltwo-degrees and three-degrees of freedom tidal volume models.

It is another object of the invention to provide noninvasive methods andassociated systems for determining pulmonary ventilation that can bereadily employed to measure pulmonary ventilation in different postureswhen awake and during sleep.

It is another object of the invention to provide noninvasive methods andassociated systems for determining pulmonary ventilation that can bereadily employed to accurately detect respiratory abnormalities.

It is yet another object of the invention to provide noninvasive methodsand associated systems for determining pulmonary ventilation that can bereadily employed to accurately detect respiratory events, such as apneasand hypopneas, during sleep.

SUMMARY OF THE INVENTION

In accordance with the above objects and those that will be mentionedand will become apparent below, the noninvasive method for measuringpulmonary ventilation, i.e. determining the tidal volume (V_(T)), of asubject, in accordance with one embodiment of the invention, generallycomprises the steps of:

(i) determining a first anatomical characteristic representing a firstlinear displacement of the subject's rib cage in a first orientation,

(ii) determining a second anatomical characteristic representing a firstlinear displacement of the subject's abdomen in the first orientation,

(iii) determining a third anatomical characteristic representing a firstaxial displacement of the subject's chest wall in the first orientation,

(iv) determining a first rib cage volume-motion coefficient representingthe first orientation of the subject,

(v) determining a first abdomen volume-motion coefficient representingthe first orientation of the subject,

(vi) determining a first chest wall volume-motion coefficientrepresenting the first orientation of the subject,

(vii) providing a first mathematical relationship that is adapted todetermine V_(T) of the subject as a function of the first, second andthird anatomical characteristics, the first rib cage volume-motioncoefficient, first abdomen volume-motion coefficient and first chestwall volume-motion coefficient, and

(viii) determining V_(T) of the subject with the first mathematicalrelationship and the first, second and third anatomical characteristics,and the first abdomen volume-motion coefficient, first abdomenvolume-motion coefficient and first chest wall volume-motioncoefficient, whereby the determined V_(T) represents the subject's V_(T)at the first orientation.

In one embodiment, the first orientation comprises an upright position.

In one embodiment, the first mathematical relationship comprises thefollowing equation

V _(T) =αΔRC+(B _(u) +εXi)ΔAb+γΔXi

wherein V_(T) represents tidal volume, ARC represents the firstanatomical characteristic, ΔAb represents the second anatomicalcharacteristic, ΔXi represents the third anatomical characteristic,B_(u) represents the value of the abdominal volume-motion coefficient(B) in the upright position, α represents the first rib cagevolume-motion coefficient, (B_(u)+εXi) represents the abdominalvolume-motion coefficient, ε represents the linear slope of therelationship of B as a function of Xi, and γ represents the first chestwall volume-motion coefficient.

In one embodiment, the first mathematical relationship comprises thefollowing equation

V _(T)=α(ΔRC)+β(ΔAb)+γ(ΔXi)

wherein V_(T) represents tidal volume, ARC represents the firstanatomical characteristic, ΔAb represents the second anatomicalcharacteristic, ΔXi represents the third anatomical characteristic, αrepresents the first rib cage volume-motion coefficient, β representsthe first abdominal volume-motion coefficient, and γ represents thefirst chest wall volume-motion coefficient.

In accordance with another embodiment of the invention, there isprovided a method for determining the tidal volume (V_(T)) of a subject,comprising the steps of:

(i) providing a first sensor system adapted to measure lineardisplacement of the subject's rib cage, said first sensor systemincluding a first transmission coil adapted to transmit at least a firstsignal and a first receive coil,

(ii) providing a second sensor system adapted to measure lineardisplacement of the subject's abdomen, said second sensor systemincluding a second transmission coil adapted to transmit at least asecond signal and a second receive coil adapted to receive said secondsignal, said first receive coil being adapted to receive said first andsecond signals,

(iii) determining a first linear displacement of the subject's rib cagein a first orientation with said first sensor system,

(iv) determining a first linear displacement of the subject's abdomen insaid first orientation with said second sensor system;

(v) determining a first axial displacement of the chest wall as afunction of said second signal transmitted by said second transmissioncoil and received by said first receive coil,

(vi) determining a first rib cage volume-motion coefficient representingsaid first orientation of the subject,

(vii) determining a first abdomen volume-motion coefficient representingsaid first orientation of the subject,

(viii) determining a first chest wall volume-motion coefficientrepresenting said first orientation of the subject,

(ix) providing a first mathematical relationship that is adapted todetermine V_(T) of the subject as a function of said first lineardisplacement of the subject's rib cage, said first linear displacementof the subject's abdomen, said first axial displacement of the chestwall, said first rib cage volume-motion coefficient, said first abdomenvolume-motion coefficient and said first chest wall volume-motioncoefficient, and

(x) determining V_(T) of the subject with said first mathematicalrelationship and said first linear displacement of the subject's ribcage, said first linear displacement of the subject's abdomen, saidfirst axial displacement of the chest wall, said first abdomenvolume-motion coefficient, first abdomen volume-motion coefficient andfirst chest wall volume-motion coefficient, whereby said determinedV_(T) represents the subject's V_(T) in said first orientation.

In accordance with another embodiment of the invention, there isprovided a method for monitoring respiration of a subject, comprisingthe steps of:

(i) determining a plurality of rib cage volume-motion coefficientsrepresenting a plurality of orientations and motions of the subject,

(ii) determining a plurality of abdomen volume-motion coefficientsrepresenting the plurality of orientations and motions of the subject,

(iii) determining a plurality of chest wall volume-motion coefficientsrepresenting the plurality of orientations and motions of the subject,

(iv) compiling a plurality of volume-motion coefficient data sets, theplurality of volume-motion data sets including a first plurality ofvolume-motion coefficient data sets representing the plurality oforientations of the subject and a second plurality of volume-motioncoefficient data sets representing the plurality of motions of thesubject, each of the first plurality of volume-motion coefficient datasets comprising a respective one of the rib cage, abdomen and chest wallvolume-motion coefficients that represent a respective one of theplurality of orientations, each of the second plurality of volume-motioncoefficient data sets comprising a respective one of the rib cage,abdomen and chest wall volume-motion coefficients that represent arespective one of the plurality of motions,

(v) providing a first mathematical relationship that is adapted todetermine the tidal volume (V_(T)) of the subject as a function of afirst anatomical characteristic representing a first linear displacementof the subject's rib cage, a second anatomical characteristicrepresenting a first linear displacement of the subject's abdomen, athird anatomical characteristic representing a first axial displacementof the subject's chest wall, and a respective one of the plurality ofthe volume-motion coefficient data sets,

(vi) substantially continuously determining the first, second and thirdanatomical characteristics over a first period of time,

(vii) monitoring the orientation of the subject,

(viii) determining when the subject is in a selective one of theplurality of orientations, and

(ix) substantially continuously determining V_(T) of the subject overthe first period of time with the first mathematical relationship andemploying the first, second and third anatomical characteristicsdetermined when the subject is in the respective one of theorientations, and the volume-motion coefficient data set of the firstplurality of volume-motion coefficient data sets that represents theselective one of the orientations.

In one embodiment, the method includes the steps of monitoring themotion of the subject, determining when the subject is exhibiting aselective one of the plurality of motions, determining V_(T) of thesubject with the first mathematical relationship and employing thefirst, second and third anatomical characteristics determined when thesubject is in the respective one of the motions, and the volume-motioncoefficient data set of the second plurality of volume-motioncoefficient data sets that represents the selective one of the motions.

In accordance with another embodiment of the invention, there isprovided a pulmonary ventilation system for monitoring respiration of asubject, comprising:

(i) means for substantially continuously determining a first anatomicalcharacteristic representing a first linear displacement of the subject'srib cage in a first orientation,

(ii) means for substantially continuously determining a secondanatomical characteristic representing a first linear displacement ofthe subject's abdomen in the first orientation,

(iii) means for substantially continuously determining a thirdanatomical characteristic representing a first axial displacement of thesubject's chest wall,

(iv) storage means adapted to store a first empirical relationshipadapted to determine at least one rib cage volume-motion coefficient,abdomen volume-motion coefficient and chest wall volume-motioncoefficient, and a second empirical relationship adapted to determine aventilation characteristic as a function of the first, second and thirdanatomical characteristics and the rib cage, abdomen and chest wallvolume-motion coefficients, and

(v) processing means for determining the ventilation parameter with thesecond empirical relationship.

In one embodiment of the invention, the system includes means foracquiring at least one base-line ventilation characteristic, and a thirdempirical relationship adapted to correlate the acquired base-lineventilation characteristic to a ventilation parameter determined withthe second empirical relationship.

In one embodiment, the first empirical relationship is adapted todetermine a plurality of the rib cage, abdomen and chest wallvolume-motion coefficients, the plurality of rib cage, abdomen and chestwall volume-motion coefficients representing a plurality of bodyorientations and motions.

In one embodiment, the storage means includes a plurality ofvolume-motion coefficient data sets, the plurality of volume-motion datasets including a first plurality of volume-motion coefficient data setsrepresenting the plurality of orientations and a second plurality ofvolume-motion coefficient data sets representing the plurality ofmotions, each of the first plurality of volume-motion coefficient datasets comprising a respective one of the rib cage, abdomen and chest wallvolume-motion coefficients that represent a respective one of theplurality of orientations, each of the second plurality of volume-motioncoefficient data sets comprising a respective one of the rib cage,abdomen and chest wall volume-motion coefficients that represent arespective one of the plurality of motions.

In one embodiment, the system includes means for detecting when thesubject is in one of the plurality of orientations or exhibiting one ofthe motions.

In one embodiment of the invention, the third empirical relationship isadapted to apply a selective one of the first and second plurality ofvolume-motion coefficient data sets representing one of the plurality oforientations or motions that corresponds to a detected subjectorientation or motion to the first empirical relationship, whereby theprocessing means determines a first ventilation parameter associatedwith the detected subject orientation or motion.

In accordance with another embodiment of the invention, there isprovided a pulmonary ventilation system for monitoring respiration of asubject, comprising:

(i) a first sensor system adapted to substantially continuouslydetermine a first linear displacement of the subject's rib cage, saidfirst sensor system including a first transmission device adapted totransmit at least a first signal and a first receive device,

(ii) a second sensor system adapted to substantially continuouslydetermine a first linear displacement of the subject's abdomen, saidsecond sensor system including a second transmission device adapted totransmit at least a second signal and a second receive device adapted toreceive said second signal, said first receive device being adapted tocontinuously receive said first and second signals,

said second signal representing a first axial displacement of thesubject's chest wall,

(iii) storage means adapted to store a first empirical relationshipadapted to determine at least one rib cage volume-motion coefficient,abdomen volume-motion coefficient and chest wall volume-motioncoefficient and a second empirical relationship adapted to determine aventilation characteristic as a function of said first lineardisplacement of the subject's rib cage, said first linear displacementof the subject's abdomen, said first displacement of the subject's chestwall, and said rib cage, abdomen and chest wall volume-motioncoefficients, and

(iv) processing means for determining said ventilation parameter withsaid second empirical relationship.

In one embodiment, each of the first and second receive devices isadapted to receive said first and second signals.

In one embodiment, the first and second receive devices compriseelectromagnetic coils.

In one embodiment, the first and second receive devices comprise Halleffect sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages will become apparent from the followingand more particular description of the preferred embodiments of theinvention, as illustrated in the accompanying drawings, and in whichlike referenced characters generally refer to the same parts or elementsthroughout the views, and in which:

FIG. 1 is a graphic illustration of isovolume lines over a range ofspinal flexion and extension or pelvic rotation;

FIG. 2 are comparisons of spirometric volume and volume estimates forfive subjects that were estimated using one, two and three degrees offreedom models;

FIG. 3 is a schematic illustration of one embodiment of a pulmonaryventilation system of the invention;

FIG. 4 is a schematic illustration of one embodiment of the pulmonaryventilation system data acquisition circuitry, according to theinvention;

FIG. 5A is a side view of a subject, showing the positioning of the dataacquisition circuitry shown in FIG. 4, according to one embodiment ofthe invention;

FIG. 5B is a schematic illustration of the geometric configurationsdefined by the points of attachment of transmission and receive coils,according to one embodiment of the invention;

FIG. 6 is a further schematic illustration of the data acquisitioncircuitry shown in FIG. 4, according to the invention;

FIG. 7 is a schematic illustration of another embodiment of a pulmonaryventilation system, according to the invention;

FIG. 8A is a graphical illustration of magnetometer derived volume vs.spirometer derived volume for non-obese subjects in three differentsleep positions, i.e. supine, right and left lateral decubituspositions, according to the invention;

FIG. 8B is a graphical illustration of magnetometer derived inspiratorytime vs. spirometer derived inspiratory time for non-obese subjects insupine, right and left lateral decubitus positions, according to theinvention;

FIG. 8C is a graphical illustration of magnetometer derived expiratorytime vs. spirometer derived expiratory time for non-obese subjects insupine, right and left lateral decubitus positions, according to theinvention;

FIG. 9A is a graphical illustration of magnetometer derived volume vs.spirometer derived volume for awake obese subjects in supine, right andleft lateral decubitus positions, according to the invention;

FIG. 9B is a graphical illustration of magnetometer derived inspiratorytime vs. spirometer derived inspiratory time for awake obese subjects insupine, right and left lateral decubitus positions, according to theinvention;

FIG. 9C is a graphical illustration of magnetometer derived expiratorytime vs. spirometer derived expiratory time for awake obese subjects insupine, right and left lateral decubitus positions, according to theinvention;

FIG. 10A is a graphical illustration of magnetometer derived volume vs.pneumotachograph derived volume for obese subjects in supine, right andleft lateral decubitus positions during a day-time nap, according to theinvention;

FIG. 10B is a graphical illustration of magnetometer derived inspiratorytime vs. pneumotachograph derived inspiratory time for obese subjects insupine, right and left lateral decubitus positions during a day-timenap, according to the invention;

FIG. 10C is a graphical illustration of magnetometer derived expiratorytime vs. pneumotachograph derived expiratory time for obese subjects insupine, right and left lateral decubitus positions during a day-timenap, according to the invention;

FIG. 11A is a graphical illustration of magnetometer and spirometerderived volumes vs. mean magnetometer and spirometer derived volumes forawake non-obese subjects, according to the invention;

FIG. 11B is a graphical illustration of magnetometer and spirometerderived volumes vs. mean magnetometer and spirometer derived volumes forawake obese subjects, according to the invention;

FIG. 12 is a graphical illustration of magnetometer and spirometerderived volumes vs. mean magnetometer and spirometer derived volumes forobese subjects during day-time sleep, according to the invention;

FIGS. 13-16 are exemplar recordings of magnetometer signals andassociated derived tidal volumes for a subject;

FIG. 17 is a graphical illustration of apnea and hypopnea indices for aplurality of monitored subjects, according to the invention;

FIG. 18 is a graphical illustration of apnea indices for a plurality ofmonitored subjects, according to the invention; and

FIG. 19 is a graphical illustration of hypopnea indices for a pluralityof monitored subjects, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to particularlyexemplified methods, systems or circuits, as such may, of course, vary.Thus, although a number of methods and systems similar or equivalent tothose described herein can be used in the practice of the presentinvention, the preferred materials and methods are described herein.

It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments of the invention only andis not intended to be limiting.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one having ordinaryskill in the art to which the invention pertains.

Further, all publications, patents and patent applications cited herein,whether supra or infra, are hereby incorporated by reference in theirentirety.

Finally, as used in this specification and the appended claims, thesingular forms “a, “an” and “the” include plural referents unless thecontent clearly dictates otherwise. Thus, for example, reference to “anapnea event” includes two or more such events; reference to “aventilation signal” includes two or more such signals and the like.

DEFINITIONS

The terms “ventilation parameter” and “ventilation characteristic”, asused herein, mean and include, a characteristic associated with therespiratory system and functioning thereof, including, withoutlimitation, total pulmonary ventilation, inspiration volume (V_(I)),expiration volume (V_(E)), breathing frequency, inspiratory breathingtime and expiratory breathing time.

The term “apnea”, as used herein, means and includes the temporarycessation of respiration or a reduction in the respiration rate.

The term “hypopnea”, as used herein, means and includes abnormallyshallow breathing or a slow respiratory rate.

The terms “respiratory system disorder”, “respiratory disorder” and“adverse respiratory event”, as used herein, mean and include anydysfunction of the respiratory system that impedes the normalrespiration or ventilation process, including, without limitation, anapnea and/or hypopnea.

The terms “patient” and “subject”, as used herein, mean and includehumans and animals.

As will be appreciated by one having ordinary skill in the art, thepresent invention is directed to noninvasive methods and associatedsystems for determining pulmonary ventilation that substantially reduceor eliminate the drawbacks and disadvantages associated withconventional methods and systems for determining pulmonary ventilation.As discussed in detail herein, one major advantage of the presentinvention is that the noninvasive methods and associated systems fordetermining pulmonary ventilation substantially reduce the accuracyerrors associated with conventional two-degrees and three-degrees offreedom tidal volume models.

A further major advantage of the present invention is that thenoninvasive methods and associated systems for determining pulmonaryventilation can be readily employed to monitor breathing in differentpositions while awake and during sleep, and accurately detectrespiratory events, such sleep apnea and hypopnea, during sleep.

As is known in the art, sleep apnea is generally defined as a temporarycessation of respiration during sleep. Obstructive sleep apnea is therecurrent occlusion of the upper airways of the respiratory systemduring sleep. Central sleep apnea occurs when the brain fails to sendthe appropriate signals to the breathing muscles to initiaterespirations during sleep. Those afflicted with sleep apnea experiencesleep fragmentation and complete cessation of respiration (orventilation) during sleep with potentially severe degrees ofoxyhemoglobin desaturation.

Sleep hypopnea is generally defined as abnormally shallow breathing or aslow respiratory rate. Hypopnea differs from apnea in that there remainssome air flow.

Monitoring breathing during sleep is the cornerstone to the diagnosisand management of various sleep disorders. The respiratory inductiveplythysmograph (RIP) is the most widely used technology in the diagnosisof sleep disorders. As indicated, RIP uses a “two-degrees-of-freedom”model to derive tidal volume (V_(T)) from changes in the circumference(i.e. cross-sectional area) of the rib cage and abdomen.

As is well known in the art, there is a poor correlation between V_(T)measured by a pneumotachograph and that measured by RIP during sleep.The inaccuracies in the RIP derived V_(T) have been attributed toslippage of the RIP belts; a factor that would alter RIP calibration,and to changes in posture during sleep. See, e.g., Whyte, et al.,“Accuracy of Respiratory Inductive Plythysmograph in Measuring TidalVolume During Sleep”, J. Appl. Physiol., vol. 71, pp. 1866-1871 (1991).

As will be appreciated by one having skill in the art, as a subjectbends forward, the abdomen contents displace the diaphragm cephalad andthis, in turn, expands the rib cage. It has been found that theoverestimation of volume due to this “displacement error” can be asgreat as 40-50% of the vital capacity. See Paek, et al., “PosturalEffects on Measurements of Tidal Volume From Body SurfaceDisplacements”, J. Appl. Physiol., vol. 68, pp. 2482-2487 (1990).Similar inaccuracies with RIP have been noted during sleep in asthmaticsubjects with nocturnal bronchospasm and paradoxical chest wall motion.See Ballard, et al., “Estimates of Ventilation from InductivePlythysmography in Sleeping Asthmatic Patients”, Chest, vol. 89, pp.840-845 (1986).

As discussed in detail above, the accuracy of “two-degrees-of-freedom”model is limited by virtue of changes in spinal flexion that canaccompany changes in posture. Indeed, it has been found that V_(T) canbe over or under-estimated by as much as 50% of the vital capacity withspinal flexion and extension.

It has, however, been found that the addition of a measure of the axialmotion of the chest wall, e.g., Δ distance between the xiphoid and thepubic symphysis (Xi), provides a third degree of freedom, which, whenemployed to determine V_(T) can reduce the error associated with the“two-degrees-of-freedom” model.

Although a “three-degrees-of-freedom” model can reduce the errorassociated with the “two-degrees-of-freedom” model, the“three-degrees-of-freedom” model is still limited in accuracy to about15% of actual ventilation in individuals who are doing freely movingpostural tasks, such as bending, lifting, sitting or standing, due tospinal flexion.

As indicated above, the most pronounced effect of spinal flexion is onthe abdominal volume-motion coefficient (β). With bending, β decreasesas the xiphi-umbilical distance decreases.

Eq. 3 has accordingly been modified as follows to incorporate the noteddependency:

V _(T)=α(ΔRC)+(β_(u) +εXi)×(ΔAb)+γ(ΔXi)  Eq. 4

where:ΔRC represents the linear displacement of the rib cage;ΔAb represents the linear displacement of the abdomen;ΔXi represents the change in the xiphi-umbilical distance from anupright position;α represents a rib cage volume-motion coefficient;β represents an abdominal volume-motion coefficient;β_(u) represents the value of the abdominal volume-motion coefficient(β) in the upright position;ε represents the linear slope of the relationship of β as a function ofthe xiphi-umbilical distance Xi;(B_(u)+εXi) represents the corrected abdominal volume-motioncoefficient; andγ represents a xiphi-umbilical volume-motion coefficient.

Equation 4 represents a “three-degrees-of-freedom” model, which nowreflects the dependence of β on the xiphi-umbilical distance.

According to the invention, tidal volume (V_(T)) can also be determinedas a function of changes in the anteroposterior dimensions of the ribcage and abdomen, as well as the axial displacement of the chest wall,i.e.

V _(T)=α(ΔRC)+β(ΔAb)+γ(ΔXi)  Eq. 5

where:ΔRC represents the linear displacement of the rib cage;ΔAb represents the linear displacement of the abdomen;ΔXi represents axial displacement of the chest wall;α represents a rib cage volume-motion coefficient;β represents an abdominal volume-motion coefficient; andγ represents a chest wall volume-motion coefficient.

According to the invention, the axial displacement of the chest wall canbe determined from various reference points. This is facilitated by theunique multi-functionality of selective coils of the invention andplacement thereof (discuss in detail below).

Thus, in one embodiment of the invention, the xiphi-umbilical distanceis measured to determine ΔXi. γ would thus represent a xiphi-umbilicalvolume motion coefficient.

In a preferred embodiment of the invention, the sternal-umbilicaldistance is measured to determine ΔXi. γ would thus represent asternal-umbilical volume motion coefficient.

In one embodiment of the invention, the values of volume-motioncoefficients α, β and γ are determined for three positions ororientations, i.e. supine, right and left lateral decubitus positions,by multiple linear regressions, as set forth in Stagg, et al.,“Computer-aided Measurement of Breath Volume and Time Components UsingMagnetometers”, J. Appl. Physiol., vol. 44, pp. 623-633 (1978); which isexpressly incorporated by reference herein. V_(T) is then calculatedafter applying the noted volume-motion coefficients to the signalsobtained in the same body position.

In one embodiment of the invention, the values of coefficients α, β andγ are determined during a plurality of motions or activities by multiplelinear regressions.

The term “volume-motion coefficient”, as used herein, thus means bothcoefficients representing body positions or orientations and motions ofa subject.

As will readily be appreciated by one having ordinary skill in the art,the “three-degrees-of-freedom” models of the invention substantiallyreduce V_(T) measurement errors associated with conventional two-degreesand three-degrees of freedom models resulting from changes in posture.Indeed, as set forth below, V_(T), inspiration volume (V_(I)) andexpiration volume (V_(E)) can be accurately measured in various postureswhile awake and during sleep using the “three-degrees-of-freedom” modelsof the invention.

As discussed in detail below, the “three-degrees-of-freedom” models ofthe invention can also be readily employed to detect adverse respirationor ventilation events, such as apneas or hypopneas.

Several embodiments of pulmonary ventilation systems of the inventionwill now be described in detail. It is, however, understood that theinvention is not limited to the system embodiments described herein.Indeed, as will be appreciated by one having ordinary skill in the art,systems and associated circuits similar or equivalent to the describedsystems can also be employed within the scope of the present invention.

In general, the ventilation systems of the invention include means forstoring an empirical relationship that is designed and adapted todetermine at least one ventilation parameter as a function of aplurality of anatomical measurements and volume-motion coefficients,means for acquiring the anatomical measurements, means for determiningthe plurality of motion coefficients, and processing means fordetermining the ventilation parameter based on the acquired anatomicalmeasurements and determined plurality of volume-motion coefficients. Ina preferred embodiment, the ventilation system further includes meansfor acquiring base-line ventilation characteristics and means forcorrelating the base-line ventilation characteristics to the ventilationparameter determined with the empirical relationship.

The ventilation systems of the invention are also preferably implementedin a compact, light-weight configuration that can be easily attached toor carried by, e.g. carrier vest, an individual being monitored.

Referring now to FIG. 3, there is shown one embodiment of a pulmonaryventilation system 20A of the invention. As illustrated in FIG. 3, thesystem 20A preferably includes data acquisition circuitry 21 and dataprocessing circuitry 22.

The system 20A also includes a power source 50, such as a battery. Inone embodiment of the invention, the system 20A is operable on 100 mAcurrent from a +/−8.0V to +/−12.0V power source 50.

According to the invention, the data acquisition circuitry 21 includesdata acquisition means, i.e. means for measuring (or sensing) changes inthe anteroposterior diameters of the rib cage and abdomen, and axialdisplacement of the chest wall. In one embodiment of the invention,which is illustrated in FIG. 4 and discussed in detail below, the dataacquisition means comprises paired electromagnetic coils 60-63.

It is, however, understood that the invention is not limited to the useof electromagnetic coils to measure changes in the anteroposteriordiameters of the rib cage and abdomen, and axial displacement of thechest wall. Indeed, various additional means and devices that can bereadily adapted to measure the noted anatomical parameters can beemployed within the scope of the invention. Such means and devicesinclude, without limitation, Hall effect sensors and electronic compasssensors.

Referring back to FIG. 3, in the illustrated embodiment, the dataprocessing circuitry 22 includes data input interface circuitry 24,which is adapted to receive data from the data acquisition circuitry 21.According to the invention, the data input interface circuitry 24facilitates data communication by and between the data acquisitioncircuitry 21 and the data processing circuitry 22.

In the illustrated embodiment, the data input circuitry 24 is incommunication with (i.e. operatively connected to) system buses 23, 26,and transmits (or directs) signals thereto. The central processing unit(CPU) 28 is also preferably operatively connected to buses 23, 26.

As further illustrated in FIG. 3, bus 26 is in communication with memorymeans (or medium) 30, which includes at least one, more preferably, aplurality of executable programs. In one embodiment of the invention,the programs include a calibration program (or routine) 32, a signalprocessing or element analysis program 36, a ventilation parameterprogram 38, and an output program 40.

According to the invention, the central processing unit 28 executesselective programs stored in the memory means 30. The data provided bythe programs, e.g. accumulated pulmonary ventilation data 42, andderived volume-motion coefficients are also preferably stored in thememory means 40.

In some embodiments of the invention, the calibration program 32comprises a routine for determining volume-motion coefficients inselective body positions and/or motions and calibrating the dataacquisition circuitry 21. In a preferred embodiment, calibration of thedata acquisition circuitry 21 is performed in a single step.

In some embodiments of the invention, the calibration program 32 is alsoadapted to “automatically” select derived volume-motion coefficients,i.e. a volume-motion coefficient data set, reflecting a specific bodyposition (or posture) or activity to calibrate the data acquisitioncircuitry 21 when a subject or patient is in the noted body position orperforming the activity.

The element analysis program 36 is designed and adapted to performelement analyses on acquired signals (representing acquired data) toreduce any extraneous noise in the signals. In one embodiment of theinvention, the element analyses include at least one Fourier analysis.The Fourier analysis, when combined with band pass filtering in thesoftware, facilitates use of the system 20A in ambulatory activities.

In a preferred embodiment, the ventilation parameter program 38 employsat least one of the “three-degrees-of-freedom” models of the inventionto determine at least one ventilation parameter, e.g., total pulmonaryventilation.

The output program 40 is designed and adapted to facilitate the visualdisplay of acquired data, which can be displayed on an output device 46,e.g. a liquid crystal display, or, as discussed below, an externaloutput device. As illustrated in FIG. 3, in the noted embodiment, theoutput device 46 is in communication and, hence, interacts with the CPU28 through data output interface circuitry 44.

According to the invention, the data output interface circuitry 44 canbe adapted to interact with external output devices, such as a personalcomputer, a printer, a monitor and the like. Communication by andbetween the data output interface circuitry 44 and external device(s)can be achieved via wired connections therebetween and/or wirelesstransmission.

As illustrated in FIG. 3, the memory means 30 preferably includes atleast one, preferably, a plurality of digital band pass filters 34.According to the invention, the digital band pass filters 34 aredesigned and employed to eliminate extraneous noise or artifactsresulting from soft tissue motion.

Referring now to FIG. 4, there is shown one embodiment of the dataacquisition circuitry 21 of the invention. As illustrated in FIG. 4, thedata acquisition circuitry 21 preferably includes a first transmissioncoil 60, a second transmission coil 62, a first receive coil 61 and asecond receive coil 63.

In a preferred embodiment of the invention, at least one of the tworeceive coils 61, 63 is adapted to receive transmissions (or processsignals) from each of the transmission coils 60, 62, i.e. dualfunctionality.

Accordingly, in one embodiment of the invention, the first transmissioncoil 60 comprises a first frequency (e.g., 8.97 kHz) transmitter coil,the second transmission coil 62 comprises a second frequency (e.g., 7kHz) transmitter coil, the first receive coil 61 comprises a firstfrequency receive coil, and the second receive coil 63 comprises afirst/second frequency (7/8.97 kHz based on the noted first and secondfrequency examples) receive coil.

As will be appreciated by one having ordinary skill in the art, the dualfunctionality of the second receive coil 63 reduces the number ofreceive coils, thereby reducing the number of attachments to a patientsimplifying system design, and reducing power requirements.

In some embodiments of the invention, each receive coil 61, 63 isadapted to receive transmissions from each of the transmission coils 60,62. As discussed in detail below, the dual functionality of both receivecoils 61, 63 enhances the accuracy of anatomical measurements determinedfrom transmissions by and between the coils 60-63.

Referring now to FIG. 5A, there is shown the positioning of the coils60-63 and data processing circuitry 22 on a subject or patient 51, inaccordance with one embodiment of the invention. As illustrated in FIG.5A, the first transmission coil 60 is positioned on the front of thesubject 51 proximate the umbilicus of the subject 51 and the firstreceive coil 61 is preferably positioned at the same axial position, buton the back of the subject 51. The second receive coil 63 is positionedon the front of the subject 51 proximate the base of the sternum of thesubject 51 and the second transmission coil 62 is located at the sameaxial position, but on the back of the subject 51.

In the illustrated embodiment, the data processing circuitry 22 isattached to the subject 51 via a belt 52.

According to the invention, the positions of the transmission coils 60,62 and receive coils 61, 63 can be reversed, i.e. transmission coil 60and receive coil 63 placed on the back of the subject 51 andtransmission coil 62 and receive coil 61 placed on the front of thesubject.

As discussed in detail below, both transmission coils 60, 62 can also beplaced on the front or back of the subject and the receive coils 61, 63can be placed on the opposite side.

As stated, in one embodiment of the invention, the second receive coil63 is adapted to receive and process signals from both the first andsecond transmission coils 60, 62, reducing the number of coils requiredto determine V_(T) (using a “three-degrees-of-freedom” model of theinvention) from six to four. This simplifies the instrumentationattached to a subject, and reduces the power requirements of the system20A.

According to the invention, the coils 60-63 can be attached to thesubject by various suitable means. In one embodiment of the invention,the coils 60-63 are attached to the subject 51 via medical tape.

Referring back to FIG. 5A, arrow 64 represents the Xi or, in thisinstance, the xiphi-umbilical distance. Arrow 65 represents the ribcage-anteroposterior (RC-AP) distance, while arrow 66 represents theabdomen-anteroposterior (Ab-AP) distance.

FIG. 5 thus illustrates the three degrees of freedom or motion (RC-AP,Ab-AP, and Xi) that are measured in accordance with the invention.According to the invention, as the subject or patient breathes, thechange in distance between each pair of coils 60, 61 and 62, 63 (denotedby arrows “65” and “66”) is sensed. The change in distance between thepaired coils corresponds to changes in voltage that is a function ofchanges in the anteroposterior distance of the rib cage (RC-AP) and theabdomen (Ab-AP).

The axial displacement of the chest wall (denoted by arrow “64”), e.g.,xiphi-umbilical distance (Xi), is also measured. In one embodiment ofthe invention, wherein coil 63 comprises a dual functionality coil, theaxial displacement of the chest wall is directly determined from sensedchanges in voltage between transmission coil 60 and receive coil 63.

In one embodiment of the invention, wherein receive coils 61 and 63comprise dual functionality coils, the axial displacement of the chestwall is similarly determined from sensed changes in voltage betweentransmission coil 60 and receive coil 63. However, in this instance, thesensed changes in voltage between transmission coil 62 and receive coil61, reflecting the distance between the points of attachment of coils 62and 61 (denoted length “67”) can be correlated with the sensed changesin voltage between transmission coil 60 and receive coil 63.

In one embodiment of the invention, receive coils 61 and 63 similarlycomprise dual functionality coils. However, in this embodiment, bothreceive coils 61, 63 are placed on the same side of the subject's bodyand the transmission coils 60, 62 are placed on the opposite side of thesubject's body.

By virtue of the placement of coils 60-63 and the dual functionality ofreceive coils 61, 63, various known empirical (e.g., trigonometric)relationships can be employed to determine the axial displacement of thechest wall based on the geometrical configurations defined by the pointsof attachment of the coils 60-63, and measured lengths therebetween (seeFIG. 5B). By way of example, if the triangle defined by coils 60, 61 and63, i.e. attachment points thereof, is deemed a right triangle, and thedistances of lengths “66” and “69” equal x and y, respectively, thechange in distance of length “64”, i.e. chest wall displacement (ΔXi)can be determined as follows:

ΔXi=Xi−((L ₆₉)²)−(L ₆₆)²))  Eq. 6

where:Xi represents the original axial chest wall distance, as measuredbetween two selective positions, e.g., xiphi-umbilical distance;L₆₉ represents the length denoted “69”; andL₆₆ represents the length denoted “66”.

If L₆₉ and L₆₆, and angle φ are known, the law of cosines can beemployed to determine the length “64” and changes thereof, i.e. chestwall displacement (ΔXi).

As will also be readily appreciated by one having ordinary skill in theart, the use of two dual functionality receiver coils, e.g., receivecoils 61, 63, and the placement thereof on one side of the subjectfacilitates simple and accurate determination of axial displacement ofthe chest wall, regardless of the axial placement of the receive coils61, 63 (provided, the receive coils 61, 63 remain substantially axiallyaligned).

The use of two dual functionality receiver coils, e.g., receive coils61, 63, and the placement thereof on one side of the subject alsofacilitates accurate determination of whether a measured displacement ofthe rib cage actually reflects true ventilation of a subject.

According to the invention (and discussed in detail below), the acquireddata representing the noted measured distances is employed by theventilation parameter program 38 of the invention to determine one ormore ventilation parameters or characteristics.

Referring back to FIG. 4, in the illustrated embodiment, a firsttransmission signal pre-processor 70 transmits a signal to the firsttransmission coil 60. Similarly, a second transmission signal processor72 transmits a signal to the second transmission coil 62.

The received signals are then processed by three channels, including afirst detection circuitry channel 80, a second detection circuitrychannel 82, and a third detection circuitry channel 84. The output ofthe individual channels is preferably transmitted to the data inputinterface circuitry 24 for subsequent processing and storage by the CPU28, in accordance with the executable programs stored in memory 30.

The data acquisition circuitry 21 also includes a flow meter 86, whoseoutput is preferably processed by a low pass filter 88 before beingtransmitted to the data input interface circuitry 24. The data providedby the flow meter 86 is employed during the calibration step, asdiscussed below.

Referring now to FIG. 6, there is shown a more detailed view of selectedcomponents of the data acquisition circuitry 21, in accordance with oneembodiment of the invention. As illustrated in FIG. 6, the dataacquisition circuitry 21 includes a first transmission signalpre-processor 70 having a first oscillator 94. In one embodiment, thefirst oscillator 94 is set to 8.97 kHz.

According to the invention, the oscillator signal is transmitted to afirst transmission channel variable gain circuit 92, which allows anoptimal gain value to be set. The gain adjusted signal is thentransmitted to a differential signal driver 90, and is then transmittedto the first transmission coil 60.

As illustrated in FIG. 6, the output from the differential signal driver90 is also transmitted to the first channel detection circuitry 80 andthe second channel detection circuitry 82, as will be discussed furtherbelow.

The second transmission signal-processor 72 operates in a similarmanner. According to the invention, the second oscillator 100 oscillatesat a pre-determined frequency, e.g., 7 kHz. The oscillator signal istransmitted to a second transmission channel variable gain circuit 98,which is independently set for an optimal gain value. The gain adjustedsignal is then transmitted to a differential signal driver 96, and isthen transmitted to the second transmission coil 62. The output of thedifferential signal driver 96 is also transmitted to the third channeldetection circuitry 84, as will be discussed further below.

The signal from the first transmission coil 60 is processed by the firstreceive coil 61 and is then transmitted to the first received signalpre-processor 74. As illustrated in FIG. 6, the first signalpre-processor 74 can be implemented with an input stage 102, a firstreceive channel variable gain 104, and a band pass filter 106.

According to the invention, the variable gain 104 can be set through thedata input interface circuitry 24 to optimize the signal-to-noise ratio.Preferably, the band pass filter 106 is set to reduce noise above andbelow 8.97 KHz.

The second received signal pre-processor 76 operates in a similarmanner. However, as indicated above, the second receive coil 63 ispreferably adapted to process two signals. The second received signalpre-processor 76 accordingly processes two signals.

As illustrated in FIG. 6, a single input stage 108 processes the twosignals and transmits the output to two channels in communication withthe input stage 108. Each channel includes a variable gain circuit110/116 and a band pass filter 112/118.

According to the invention, the separate gain controls 92, 98, 104, 110,116 are preferably optimized to increase the signal-to-noise ratio. Thegain controls 92, 98 for the transmitted signal can also be optimized tominimize power requirements.

Since the gain for the transmitted signal can be changed independentlyof the gain of the receiver channel, the signal-to-noise ratio can beimproved, while minimizing the magnetic field exposure at skin surfaceof the patient.

According to the invention, the band pass filters 106, 112, 118 areadapted to minimize interference from extraneous magnetic fields andnoise sources. As illustrated in FIG. 6, the output from the firstreceived signal preprocessor 74 is transmitted to the first channeldetection circuitry 80. The circuitry 80 preferably includes a firstdetector 120, which is set to the frequency established by the firstoscillator 94. The output of the first detector 120 is transmitted to alow pass filter 122 and an absolute value circuit 124. The signal isthen transmitted to the data input interface circuitry 24 for processingby the CPU 28.

The second channel detection circuitry 82 operates in a similar manner.The second detector 128 is set to the frequency established by the firstoscillator 94 and processes the signal from the second receive channelband pass filter 112. The second channel detection circuitry 82 includesa low pass filter 130 and an absolute value circuit 132 to produce adata signal that is also transmitted to the CPU 28 for processing, inaccordance with the executable programs stored in memory 30.

The third channel detection circuitry 84 is set to the frequencyestablished by the second oscillator 100 and processes the signal fromthe third receive channel band pass filter 118. The third receivechannel detection circuitry 84 also includes a low pass filter 138 andan absolute value circuit 140.

Processing associated with the executable programs stored in memory 30will now be described in detail.

According to the invention, the calibration program or routine 32determines calibration coefficients, i.e. volume-motion coefficients,throughout a range of body positions and activities. Calibrationcoefficients are then derived for specific activities or body postures,i.e. sets of calibration coefficients. These different sets ofcalibration coefficients are then applied to selected regions of theacquired data set.

The calibration routine 32 thus allows the user to employ volume-motioncoefficients from different segments of the data set (e.g., sitting,standing, walking, etc.). These coefficients can then be applied to thedata set to construct spirograms of volume over time.

In some embodiments of the invention, the calibration program 32 is alsoadapted to “automatically” select derived volume-motion coefficients,i.e. a volume-motion coefficient data set, reflecting a specific bodyposition (or posture) or activity when a subject or patient is in thenoted body position or performing the activity. In the notedembodiments, the calibration program 32 would automatically select avolume-motion coefficient data set in response to a transmitted bodyposture-motion signal.

As will be appreciated by one having ordinary skill in the art, varioussensors can be employed within the scope of the invention to sense andtransmit the body posture-motion signal, e.g., 3-axis accelerometer. Inone embodiment of the invention, the calibration program 32 isresponsive to a signal reflecting the axial displacement of the chestwall, e.g., change in the sternal or xiphi-umbilical distance.

As discussed in detail in the Examples section, the calibration routine32 facilitates the accurate determination of the volume of air inhaledand exhaled; the volume, i.e. V_(T), being determined from the sum ofthree signals (the changes in the axial dimensions of theanteroposterior diameter of the rib cage (RC) and abdomen (Ab) and thechanges in the axial dimensions of the anterior chest wall).

In accordance with one embodiment of the invention, the calibrationmaneuver includes having a subject breath through a flow meter, e.g.,flow meter 86, for 1-2 minutes at varied tidal volumes and bodypositions.

In some embodiments of the invention, the calibration program 32 is alsoused to adjust the variable gain elements 92, 98, 104, 110, and 116 foroptimum signal levels.

As indicated above, in one embodiment, the element analysis program 36is adapted to perform element analyses on acquired signals (representingacquired data) to reduce any extraneous noise in the signals. In oneembodiment of the invention, the element analyses includes at least oneFourier analysis, which, combined with band pass filtering in thesoftware, facilitates use of the system 20A in ambulatory activities.

In one embodiment of the invention, the ventilation parameter program 38correlates the “three-degrees-of-freedom” data with the flow meter data.The parameter program 38 then employs the data to create correlationparameters for determining ventilation parameters or characteristics,including end-expiratory, lung volume, breathing frequency, totalpulmonary ventilation, inspiratory breathing time, expiratory breathingtime, and total breathing time. These parameters can then be displayedon an integral visual output device or transmitted wirelessly to anexternal receiver and/or display device.

Referring now to FIG. 7, there is shown another embodiment of thepulmonary ventilation system 20B of the invention. As illustrated inFIG. 7, the system 20B is similar to system 20A. However, in thisembodiment the power source 50 is external to the data processingcircuitry 22. Further, the pulmonary ventilation system 20B includes aremovable memory 160 (e.g., a flash memory card), which is employed tostore the accumulated data 42. The accumulated data can then betransferred to a receiving station, such as a personal computer 162.

In the illustrated embodiment, the personal computer 162 includes acentral processing unit 164 and a set of input/output devices 166, whichcommunicate via bus 168. The system 20B also includes memory medium 170,which is in communication with bus 168. As illustrated in FIG. 7, thememory medium 162 includes the element analysis program, ventilationparameter program and digital band pass filters 34.

As will be readily appreciated by one having ordinary skill in the art,system 20B facilitates processing of the accumulated data 42 with aseparate device.

EXAMPLES

The following examples are given to enable those skilled in the art tomore clearly understand and practice the present invention. They shouldnot be considered as limiting the scope of the invention, but merely asbeing illustrated as representative thereof.

Example 1 Subjects

Thirty (30) subjects were selected to assess the accuracy of the“three-degrees-of-freedom” model of the invention. As reflected in Table1, ten (10) healthy subjects with no known history of sleep-disorderedbreathing and ten (10) obese subjects were studied while awake in thesupine, right and left lateral decubitus positions. Ten (10) obesesubjects with obstructive sleep apnea (obese sleeping subjects) werestudied while unrestrained during a daytime nap.

TABLE 1 Non-Obese Obese Obese Subjects Awake Awake Daytime SubjectsSubjects Nap⁺ Subject (BMI) (BMI) (BMI) 1 20 52 51 2 30 45 61 3 27 61 494 28 49 40 5 27 59 34 6 29 40 38 7 23 34 51 8 27 38 39 9 21 50 56 10 2539 38 Mean ± SD 25.7 ± 3.4 46.7 ± 9.1 45.7 ± 1.0 BMI: Body Mass Index

The non-obese, awake subjects comprised seven (7) males and three (3)females, age 28-49 yr. (mean age 30.2 yr). The obese, awake subjectscomprised six (6) males and four (4) females, age 24-64 yr. (mean age44.4 yr). The obese, napping subjects comprised five (5) males and five(5) females, age 28-60 yr. (mean age 42.2 yr).

Device and Measurements

The anteroposterior displacements of the rib cage and abdomen, as wellas the axial displacements of the chest wall (i.e. Xi) were measuredusing a light-weight, portable pulmonary ventilation system of theinvention (also referred to herein as a magnetometer system or device)using the “three-degrees-of-freedom” model set forth in Eq. 5 above.Signals were sampled at 20 Hz and stored to compact flash memory.

Calibration was performed with subjects in the supine, right lateral andleft lateral decubitus positions, as set forth in Paek, et al.,“Postural Effects on Measurements of Tidal Volume From Body SurfaceDisplacements”, J. Appl. Physiol., vol. 68, pp. 2482-2487 (1990); whichis expressly incorporated by reference herein.

Protocol

The non-obese and obese awake subjects were studied while in the supine,right lateral and left lateral decubitus positions (hereinafter “testpositions”). After calibration, each subject breathed through amouthpiece connected to a spirometer (PK Morgan Ltd®). Subjects wereinstructed to take breaths ranging from 0.5 to 2.5 L. At least 15breaths (average 25.6 breaths) of varied volumes were obtained from eachsubject in each position. Data was simultaneously collected using aventilation or magnetometer system of the invention, such as system 20Bdescribed above.

The daytime nap studies were done using a tight-fitting facemaskequipped with a pneumotachograph applied to cover the nose and mouth.Subjects were observed while sleeping in a quiet room. Each subject wasmonitored continuously and changes in position were recorded. Data wascollected simultaneously using the magnetometer system.

Statistical Analysis

The coefficient of determination (R²) was calculated for V_(T), T_(I),and T_(E) derived from the simultaneous spirometer or pneumotachographand magnetometer signals using a linear correlation model. This analysiswas performed for individual subjects in each position and for pooleddata from all subjects in all positions. The mean percent differences (%difference) between the spirometer or pneumotachograph and magnetometerderived measurements (V_(T), T_(I) and T_(E)) were also calculated.

Both absolute and mathematical differences were calculated and used toassess correlation and agreement (see below). T_(I) and T_(E)correlations were calculated for the daytime nap studies using 65randomly selected breaths from each study.

The methods set forth in Bland, et al., “Statistical Methods forAssessing Agreement Between the Two Methods of Clinical Measurement”,Lancet, vol. 1, pp. 307-310 (1986), which is incorporated by referenceherein, were used to assess the agreement between measurements obtainedfrom the spirometer and magnetometer or pneumotachograph andmagnetometer.

Results

As discussed in detail below, there were significant correlationsbetween the spirometer and magnetometer measurements of V_(T), T_(I) andT_(E) during wakefulness for the non-obese and obese subjects. For thenon-obese awake subjects (n=10), the R² values for V_(T), T_(I), andT_(E) for each subject in each of the test positions were determined(see Tables 2 and 3A and 3B below):

TABLE 2 Non-Obese Awake Subjects Obese Awake Subjects Mean % Mean %Subject Diff* ± SD R² Subject Diff* ± SD R² 1 12.2 ± 12% 0.94 11  8.7 ±7% 0.96 2  8.1 ± 8% 0.97 12  9.7 ± 7% 0.94 3  7.5 ± 6% 0.97 13  6.6 ± 3%0.98 4 10.2 ± 9% 0.95 14 13.8 ± 7% 0.84 5 11.8 ± 9% 0.90 15 12.3 ± 9%0.91 6  7.6 ± 7% 0.98 16  7.0 ± 4% 0.98 7 13.5 ± 9% 0.90 17  8.8 ± 6%0.92 8 12.1 ± 8% 0.97 18  6.4 ± 4% 0.97 9 13.8 ± 9% 0.87 19  7.3 ± 6%0.94 10 10.5 ± 6% 0.92 20 10.5 ± 11% 0.92 Mean ± SD 10.6 ± 9% 0.94 Mean± SD  9.0 ± 7% 0.95 *Absolute differences

TABLE 3A Non-Obese Awake Subjects T₁ T₂ Mean % Mean % Subject Diff* ± SDR² Diff* ± SD R² 1 11.4 ± 11% 0.86  8.5 ± 9% 0.87 2  9.3 ± 9% 0.72  8.0± 7% 0.83 3  8.1 ± 4% 0.89  9.2 ± 4% 0.91 4 10.5 ± 8% 0.61 10.6 ± 8%0.78 5  5.7 ± 5% 0.96  7.4 ± 6% 0.89 6  9.6 ± 9% 0.84  7.3 ± 6% 0.92 712.7 ± 9% 0.89  8.5 ± 5% 0.97 8 15.0 ± 14% 0.83  9.0 ± 7% 0.95 9  6.5 ±7% 0.87  7.6 ± 7% 0.88 10 10.8 ± 8% 0.88  9.7 ± 8% 0.87 Mean ± SD  9.7 ±9% 0.88  8.6 ± 8% 0.91

TABLE 3B Obese Awake Subjects T₁ T₂ Mean % Mean % Subject Diff* ± SD R²Diff* ± SD R² 11 9.5 ± 9% 0.88 6.3 ± 6% 0.93 12 8.5 ± 8% 0.77 7.5 ± 6%0.90 13 7.2 ± 6% 0.87 7.4 ± 7% 0.90 14 3.5 ± 3% 0.91 3.6 ± 3% 0.96 158.4 ± 7% 0.87 7.7 ± 6% 0.83 16 5.4 ± 5% 0.91 5.4 ± 5% 0.95 17 6.4 ± 5%0.80 5.7 ± 6% 0.89 18 8.3 ± 7% 0.94 5.0 ± 5% 0.97 19 8.1 ± 6% 0.92 7.5 ±6% 0.97 20 5.9 ± 4% 0.91 4.8 ± 3% 0.95 Mean ± SD 7.5 ± 7% 0.96 6.4 ± 6%0.97 *Absolute differences

For pooled data of all subjects in all test positions, a total of 645breaths were analyzed. The R² values for V_(T), T_(I) and T_(E) were0.94, 0.88, and 0.91, respectively (see FIGS. 8A, 8B, and 8C).

The absolute mean % differences for V_(T), T_(I) and T_(E) for eachsubject in each of the test positions are also shown in Tables 2, 3A and3B. As reflected in Tables 2, 3A and 3B, the absolute mean % differencesfor V_(T), T_(I) and T_(E) from pooled data were 10.6±9%, 9.7±9%, and8.6±8% (means±SD), respectively.

For the obese awake subjects (n=10), similar results were obtained. TheR² values for V_(T), T_(I) and T_(E) were similarly determined and areset forth in Tables 2 and 3.

For pooled data of all subjects in all positions, a total of 892 breathswere analyzed. The R² values for V_(T), T_(I), and T_(E) were 0.95,0.96, and 0.97, respectively (see FIGS. 9A, 9B, and 9C).

The absolute mean % differences for V_(T), T_(I) and T_(E) for eachsubject in each of the test positions are similarly shown in Tables 2and 3. As also reflected in Tables 2, 3A and 3B, absolute mean %differences for V_(T), T_(I) and T_(E) from pooled data were 9.0±7%,7.5±7%, and 6.4±6% (mean±SD), respectively.

There were also significant correlations between the pneumotachographand the magnetometer measurements during daytime sleep for the obesesubjects. The R² values for V_(T), T_(I) and T_(E) for each of the obesesleeping subjects were determined and are set forth in Table 4 below.

TABLE 4 Obese Subjects During Daytime Nap V_(T) T_(I) T_(E) Mean % Mean% Mean % Diff* Diff* Diff* Subject % SD R² % SD R² % SD R² 21  9.1 ± 8%0.93 13.5 ± 7% 0.88  7.6 ± 4% 0.94 22  6.2 ± 5% 0.92 11.2 ± 14% 0.8910.3 ± 7% 0.90 23  5.7 ± 5% 0.80  7.1 ± 7% 0.92  5.2 ± 4% 0.84 24 15.8 ±12% 0.87  7.1 ± 5% 0.95  5.3 ± 4% 0.98 25  8.9 ± 7% 0.91  6.4 ± 5% 0.95 6.5 ± 5% 0.90 26  7.6 ± 6% 0.96  7.2 ± 6% 0.96  5.8 ± 5% 0.97 27 13.9 ±11% 0.81  9.9 ± 6% 0.94  6.8 ± 5% 0.97 28  5.6 ± 4% 0.89  9.6 ± 7% 0.94 6.4 ± 4% 0.95 29 12.2 ± 9% 0.93  8.9 ± 6% 0.87  7.4 ± 5% 0.95 30 10.8 ±10% 0.73  8.5 ± 6% 0.89  5.1 ± 4% 0.90 Mean ± SD  9.1 ± 8% 0.94  8.9 ±8% 0.95  6.6 ± 5% 0.95 *Absolute differences

For V_(T), a total of 3861 breaths were analyzed. For T_(I) and T_(E),65 randomly selected breaths were analyzed from each subject (n=650breaths). The R² values for V_(T), T_(I) and T_(E) were 0.94, 0.95, and0.95, respectively (see FIGS. 10A, 10B, and 10C).

The absolute mean % differences for V_(T), T_(I), and T_(E) forindividual subjects are also shown in Table 4. As reflected in Table 4,the absolute mean % differences for V_(T), T_(I) and T_(E) from pooleddata were 9.1±8%, 8.9±8%, and 6.6±5%, respectively.

Using the Bland, et al. methods, agreement between the magnetometer andspirometer or pneumotachograph values for V_(T) (V_(T Mag) andV_(T Spiro) or V_(T Pn)), T_(I) (T_(I Mag) and T_(I Spiro) or T_(I Pn)),and T_(E) (T_(E Mag) and T_(E Spiro) or T_(E Pn)) for subjects in theabove different groups was assessed. The mathematical mean differences(d) between V_(T Spiro) and V_(T Mg), and the 95% confidence intervals(reflecting bias) for the awake non-obese and obese subjects aregraphically illustrated in FIGS. 11A and 11B.

A similar graphical illustration for the obese sleeping subjects usingthe V_(T Mag) and V_(T Pn) data is shown in FIG. 12.

The mathematical mean difference for the non-obese awake subjects in allpositions was 26.4 ml with a standard error (SE) of 6.8 ml. The meansfor V_(T Spiro) and V_(T Mag) for this data set were 1177.3 ml and1150.9 ml, respectively.

For the obese awake subjects, the mathematical mean difference was −13.5ml with a SE of 4.0 ml. The means for V_(T Spiro) and V_(T Mag) for thisdata set were 868.0 ml and 881.5 ml, respectively.

The mathematical mean difference between V_(T Pn) and V_(T Mag) for theobese sleeping subjects was −17.7 ml with a SE of 1.0 ml. Mean V_(T Pn)and V_(T Mag) for this group were 460.7 ml and 478.4 ml, respectively.

The limits of agreement (mean±95% confidence intervals) are shown inTable 5 below.

TABLE 5 Limits of Magnetometer Mean Diff Agreement Group Mean (d) ± SD(d) ± 95% CI Non-Obese V_(T) (ml) 1150.9    26.4 ± 173.0 −319.6 to+372.4 Awake T_(I) (sec) 1.80  −0.06 ± 0.20  −0.46 to +0.34 SubjectsT_(E) (sec) 1.93  +0.06 ± 0.20  −0.34 to +0.46 Obese V_(T) (ml) 881.5 −13.5 ± 119.0 −251.5 to +224.5 Awake T_(I) (sec) 1.45  −0.03 ± 0.12 −0.27 to +0.21 Subjects T_(E) (sec) 1.76  +0.02 ± 0.13  −0.24 to +0.28Obese V_(T) (ml) 478.4  −17.7 ± 59.5 −136.7 to +101.3 Subjects T_(I)(sec) 1.39 +0.001 ± 0.14  −0.28 to +0.28 During T_(E) (sec) 1.92 +0.003± 0.15  −0.30 to +0.30 Daytime Nap (d): Mathematical differences CI:Confidence Intervals

The limits of agreement for T_(I) and T_(E) during wakefulness and sleepare also summarized in Table 5. It can be seen that the limits ofagreement for V_(T) are clinically acceptable (e.g. ±20-30% of V_(T)).As evidenced by the graphical illustrations shown in FIGS. 11A, 11B and12, most of the data is aggregated around zero with a minor percentagescattered peripherally. This scattering of data over the higher rangesof V_(T) increases the limits of agreement.

This peripheral scatter is primarily due to three factors. First, therange of V_(T) studied was wide during wakefulness (e.g., for group 1,V_(T) ranged from 250 to 4100 ml), as the subjects were instructed totake occasional deep breaths. The inaccuracies over this range of volumemay be due to the lesser range of V_(T) encountered during sleep.

Second, some of the measurements that were scattered peripherally mayrepresent accidental manual errors in measurements from the spirometeror pneumotachograph. Third, the effect of the facemask on V_(T) may haveincreased V_(T) in our sleeping subjects.

Nonetheless, as will be appreciated by one having ordinary skill in theart, the agreements and bias are accurate enough to be clinicallyrelevant.

Example 2 Subjects

The following study was conducted to assess the accuracy of the“three-degrees-of-freedom” model of the invention and a pulmonaryventilation or magnetometer system employing the subject model to detectapneas and hypopneas during sleep.

As reflected in Table 6, fifteen (15) subjects (10 males and 5 females)with variable degrees of clinical suspicion for obstructive sleep apnea(OSA) were selected for the study. The mean age for the group was45.47±12.5 years (mean±SD). The mean body mass index for the group was35.00±5.7 kg/m².

TABLE 6 Body Mass Index Subject Age (BMI) 1 50 32 2 71 28 3 34 33 4 3931 5 30 36 6 48 41 7 40 29 8 49 28 9 74 43 10 34 39 11 44 34 12 38 33 1339 46 14 46 41 15 46 31 Mean ± SD 35.00 ± 5.7 45.47 ± 12.5

Device and Measurements

The anteroposterior displacements of the rib cage and abdomen, as wellas the axial displacements of the chest wall (i.e. Xi) were similarlymeasured using a light-weight, portable pulmonary ventilation system ofthe invention (also referred to herein as a magnetometer system ordevice) using the “three-degrees-of-freedom” model set forth in Eq. 5above.

Two pairs of electromagnetic coils, each ˜½″ in diameter, were employedto measure three degrees of chest wall motion. The coils were attachedas shown in FIG. 3.

Signals were sampled at 20 Hz and stored to compact flash memory. Apneumotachograph was also attached to the magnetometer system forcalibration.

Calibration was performed with subjects in the supine, right lateral andleft lateral decubitus positions. Each subject was instructed to breaththrough the pneumotachograph with breaths of varied depth for greaterthan 1 min. Free movements of the upper and lower extremities were alsoencouraged during the calibration process (i.e. flexion and extension ofthe hip, knee, elbow and shoulder joints).

Multiple linear regression of the change in each chest wall dimension,with the corresponding tidal volume integrated from thepneumotachograph, was performed to obtain the volume-motion coefficientsof Eq. 5.

Protocol

After calibration in the supine, right lateral and left lateraldecubitus positions (hereinafter “test positions”), continuous recordingof the magnetometer signals was performed throughout a 12 leadpolysomnography (PSG).

Exemplar recordings of magnetometer signals and associated derived tidalvolume are shown in FIGS. 13-16; FIG. 13 reflecting quiet breathing of asubject during sleep with no apneas or hypopneas, FIG. 14 reflectingsleep onset of a subject (denoted generally “200”) followed by hypopneas(denoted generally “202”) and apneas (denoted generally “204”), FIG. 15reflecting recurrent hypopneas (denoted generally “206”), and FIG. 16reflecting recurrent apneas (denoted generally “208”).

Scoring of the magnetometer data was conducted in 30 second intervals.No oxygen saturation or EEG arousals were employed in the scoringprocess.

Scoring comprised polysomnography (PSG) scores and the magnetometersystem signals (or scores). Apnea and hypopnea indices (AHI), apneaindices (AI) and hypopnea indices (HI) were also recorded.

A respiratory event (i.e. apnea or hypopnea) or occurrence was definedas ≧50% reduction in V_(T) with a trend that was sustained for ≧10seconds. Apneas were identified when the volume tracing was almost flat(minor oscillations were ignored). Hypopneas were identified when thevolume could be measures (i.e. >120 cc).

Statistical Analysis

The mean apnea, hypopnea count and indices were compared using thestandard t-test. Means±standard deviation (mean±SD) was also determined.

Results

No significant differences were noted between the PSG scores and the MTGscores for the apnea and hypopnea index (AHI), apnea index (AI) andhypopnea index (HI). As reflected in Table 7, the mean±SD AHI for thePSG and MTG data were 38.9±30.5 and 42.5±27.9, respectively.

TABLE 7 PSG MTG Subject A + H (n) AHI (n/hr) A + H (n) ANI (n/hr) 1 13122.6 161 27.5 2 315 48.0 36.3 55.3 3 15 2.3 10 1.5 4 138 51.1 178 65.9 5189 30.9 222 36.4 6 102 21.0 135 27.8 7 48 15.0 59 18.4 8 149 28.7 21541.3 9 352 73.3 397 82.7 10 384 75.3 418 81.9 11 395 92.4 321 75.1 12 51.1 13 2.8 13 25 5.4 82 17.7 14 144 31.4 143 31.2 15 132 85.6 112 72.6Mean ± SD 168.3 ± 133.0 38.9 ± 30.5 188.6 ± 133.2 42.5 ± 27.9 A + H:apnea and hypopnea count

Referring now to FIG. 17, there is shown a graphical illustration of theapnea and hypopnea indices (AHI) for all fifteen subjects. The dashedlines in FIG. 9 represent the limits of severity of sleep apnea, i.e.AHI 5-15 deemed mild sleep apnea, >15-30 deemed moderate sleep apnea,and >30 deemed severe sleep apnea.

As illustrated in FIG. 17, there was significant agreement in severitybetween PSG and MTG scores.

As reflected in Table 8, the mean±SD AI for the PSG and MTG data were13.6±13.3 and 13.8±10.0, respectively.

TABLE 8 PSG MTG Subject Apnea (n) AI (n/hr) Apnea (n) AI (n/hr) 1 30 5.248 8.2 2 98 14.9 121 18.4 3 3 0.5 3 0.5 4 54 20.0 70 25.9 5 63 10.3 8614.1 6 14 2.9 35 7.2 7 12 3.7 10 3.1 8 61 11.8 87 16.7 9 50 10.4 92 19.210 108 21.2 153 30.0 11 143 33.5 135 31.6 12 0 0 0 0 13 10 2.2 42 9.1 1491 19.9 38 8.3 15 73 47.4 22 14.3 Mean ± SD 54 ± 43.1 13.6 ± 13.3 62.8 ±48.4 13.8 ± 10.0

As illustrated in FIG. 18, there was no statistically significantdifference in the mean apnea index (AI) between the MTG and PSG scores.

Referring now to Table 9, the mean±SD HI for the PSG and MTG data were25.6±20.3 and 28.8±19.5, respectively.

TABLE 9 PSG MTG Subject Hypopnea (n) HI (n/hr) Hypopnea (n) HI (n/hr) 1101 17.4 113 19.3 2 217 33.1 242 36.9 3 12 1.8 7 1.0 4 84 31.1 108 40.05 126 20.6 136 22.3 6 88 18.1 100 20.6 7 36 11.3 49 15.3 8 88 16.9 12824.6 9 302 62.9 305 63.5 10 276 54.1 265 51.9 11 252 58.9 186 43.5 12 51.1 13 2.8 13 15 3.2 40 8.6 14 53 14.6 105 22.9 15 59 38.3 90 58.4 Mean± SD 114.3 ± 99.5 25.6 ± 20.3 125.8 ± 89.3 28.8 ± 19.5

As illustrated in FIG. 19, there was similarly no statisticallysignificant difference in the mean hypopnea index (HI) between the MTGand PSG scores.

There was also significant agreement in the severity of sleep apnea(i.e. AHI 5-15 deemed mild, >15-30 deemed moderate, and >30 deemedsevere) between the MTG and PSG scores. As reflected in FIG. 13, onlytwo subjects had a change in the AHI from mild (i.e. 5.4/hr.) tomoderate (i.e. 17.7/hr.), i.e. subject #13 (shown in parentheses) andfrom moderate (i.e. 28.7/hr.) to severe (41.3/hr.), i.e. subject #6,respectively.

This study thus demonstrates that the “three-degrees-of-freedom” modelof the invention and a magnetometer system employing the subject modelcan accurately and readily detect apneas and hypopneas during sleep. Thesystem can also readily establish the severity of sleep apnea.

As will be appreciated by one having ordinary skill in the art, the“three-degrees-of-freedom” models of the invention and ventilation (ormagnetometer) systems employing the models can be readily employed toaccurately determine ventilation parameters or characteristics,including total pulmonary ventilation, breathing frequency, inspiratorybreathing time, expiratory breathing time and total breathing time. Thenoted ventilation parameters can be employed to identify normalbreathing patterns at rest (while awake and during sleep) and duringactivities, with changes in posture, with exposure to pollutants, or toidentify abnormal breathing patterns or respiratory events, such asthose presented with respiratory dyskinesia, impending respiratoryfailure, exacerbations of emphysema, asthma and other forms of lungdisease.

The ventilation parameters can also be readily employed to detect andcharacterize obstructive and central apneic episodes in adults andinfants during sleep, calculate flow volume loops during exercise andsleep, characterize breathing patterns in individuals with undiagnosedcauses of dyspnea, and determine the effects of air toxins on pulmonaryand cardiovascular health.

Without departing from the spirit and scope of this invention, one ofordinary skill can make various changes and modifications to theinvention to adapt it to various usages and conditions. As such, thesechanges and modifications are properly, equitably, and intended to be,within the full range of equivalence of the following claims.

What is claimed is:
 1. A ventilation system for monitoring respirationof a subject, comprising: means for substantially continuouslydetermining a first anatomical characteristic representing a firstlinear displacement of the subject's rib cage in a first orientation;means for substantially continuously determining a second anatomicalcharacteristic representing a first linear displacement of the subject'sabdomen in said first orientation; means for substantially continuouslydetermining a third anatomical characteristic representing a first axialdisplacement of the subject's chest wall; storage means adapted to storea first empirical relationship adapted to determine at least one ribcage volume-motion coefficient, abdomen volume-motion coefficient andchest wall volume-motion coefficient, and a second empiricalrelationship adapted to determine a ventilation characteristic as afunction of said first, second and third anatomical characteristics andsaid rib cage, abdomen and chest wall volume-motion coefficients; andprocessing means for determining said ventilation parameter with saidsecond empirical relationship.
 2. The system of claim 1, wherein saidsystem includes means for acquiring at least one base-line ventilationcharacteristic, and a third empirical relationship adapted to correlatesaid acquired base-line ventilation characteristic to a ventilationparameter determined with said second empirical relationship.
 3. Thesystem of claim 2, wherein said third empirical relationship is storedin said storage means.
 4. The system of claim 2, wherein said firstempirical relationship is adapted to determine a plurality of said ribcage, abdomen and chest wall volume-motion coefficients, said pluralityof rib cage, abdomen and chest wall volume-motion coefficientsrepresenting a plurality of body orientations and motions.
 5. The systemof claim 4, wherein said storage means includes a plurality ofvolume-motion coefficient data sets, said plurality of volume-motiondata sets including a first plurality of volume-motion coefficient datasets representing said plurality of orientations and a second pluralityof volume-motion coefficient data sets representing said plurality ofmotions, each of said first plurality of volume-motion coefficient datasets comprising a respective one of said rib cage, abdomen and chestwall volume-motion coefficients that represent a respective one of saidplurality of orientations, each of said second plurality ofvolume-motion coefficient data sets comprising a respective one of saidrib cage, abdomen and chest wall volume-motion coefficients thatrepresent a respective one of said plurality of motions.
 6. The systemof claim 5, wherein said system includes means for detecting when thesubject is in one of said plurality of orientations or exhibiting one ofsaid motions.
 7. The system of claim 6, wherein said third empiricalrelationship is adapted to apply a selective one of said first andsecond plurality of volume-motion coefficient data sets representing oneof said plurality of orientations or motions that corresponds to adetected subject orientation or motion to said first empiricalrelationship, whereby said processing means determines a firstventilation parameter associated with said detected subject orientationor motion.
 8. A ventilation system for monitoring respiration of asubject, comprising: a first sensor system adapted to substantiallycontinuously determine a first linear displacement of the subject's ribcage, said first sensor system including a first transmission deviceadapted to transmit at least a first signal and a first receive device;a second sensor system adapted to substantially continuously determine afirst linear displacement of the subject's abdomen, said second sensorsystem including a second transmission device adapted to transmit atleast a second signal and a second receive device adapted to receivesaid second signal, said first receive device being adapted tocontinuously receive said first and second signals, said second signalrepresenting a first axial displacement of the subject's chest wall;storage means adapted to store a first empirical relationship adapted todetermine at least one rib cage volume-motion coefficient, abdomenvolume-motion coefficient and chest wall volume-motion coefficient and asecond empirical relationship adapted to determine a ventilationcharacteristic as a function of said first linear displacement of thesubject's rib cage, said first linear displacement of the subject'sabdomen, said first axial displacement of the subject's chest wall, andsaid rib cage, abdomen and chest wall volume-motion coefficients; andprocessing means for determining said ventilation parameter with saidsecond empirical relationship.
 9. The system of claim 8, wherein each ofsaid first and second receive devices is adapted to receive said firstand second signals.
 10. The system of claim 8, wherein said first andsecond receive devices and said first and second receive devicescomprise magnetometer coils.
 11. The system of claim 8, wherein saidfirst and second receive devices and said first and second receivedevices comprise Hall effect sensors.